1. Field of the Invention.
This invention relates to the art of electrical acceleration switches, and more particularly concerns a miniature acceleration switch of the type having mass movable against spring bias in a housing to contact a switch terminal.
2. Description of the Prior Art.
A typical prior miniature acceleration switch of the type dealt with in this application is shown in FIG. 1. Parts are broken away to show internal parts. A switch 10 has a circuit lead 12 connected to a closed circular end wall 14 at one end of a cylindrical shell 16. A disk-like header 18, has an annular flange 20 welded to an annular flange 22 at the other end of the shell 16. The header 18 has an electrically conductive outer ring portion 24 and a central circular insulator 26. Another circuit lead 28 extends through the center of the insulator 26 and a tip 29 of the lead 28 projects slightly into a closed cylindrical chamber 30. A cylindrical mass 32 made of electrically conductive material is slidable axially in the chamber 30. The mass 32 has a cylindrical end section 34 whose diameter is less than that of a cylindrical end section 36. A cylindrical coil spring 38 is disposed axially in the chamber 30. One end 37 of the spring 38 bears on a flat inner annular side 39 of the header ring 24. The side 39 is coplanar with the adjacent annular side of the insulator 26. The other end of the spring 38 bears against an annular shoulder 40 at the junction of the sections 34 and 36 of the mass 32. The end section 34 of the mass 32 extends axially for its entire length into the coil spring 38. Flat circular end 41 of the mass 32 is normally spaced from the lead tip 29 by the load or bias in the spring 38. When the switch is accelerated in any direction, so that there is a component of sufficient force axial of the mass 32 directed toward the header 18, the mass 32 will move against the bias of the spring 38 to contact the lead tip 29 and connect leads 12 and 28 in a direct electric circuit including the shell 16, and the mass 32.
It has been found in practice that the type of acceleration switch shown in FIG. 1 is not 100% reliable in operation, for a number of reasons. For example:
1. There is often a tendency for the spring coil turns to slip or twist laterally in the space between a flat end 41 of the mass 32 and the header 18 or to slide laterally at the side 39 of the header 18 to stop movement of the mass 32 and to prevent closing of the switch by preventing contact of the mass 32 with the lead tip 29.
2. There is a tendency for turns of the spring 38 to slide radially and axially over one another laterally of the mass section 36 to twist on each other and thereby to jam the mass 32 in place and prevent its axial movement.
3. The operating point of the switch may vary unacceptably from a specified value due to the friction between the inside diameter of the mass 34 and the coils of the spring 38 as the mass moves toward the lead tip 29.
4. The switch has a limited shelf life because it is unsealed. If it is stored for long periods, the insulator 26, often made of plastic, deteriorates and the particles thereof, may break off inside the chamber 30 and obstruct the mass 32 from moving or may lodge between the header 18 and the adjacent end 41 of the mass 32 to prevent the switch from closing.
5. The air in the unsealed switch and any air which leaks into the switch contains moisture which in turn causes corrosion to develop on the inside walls of the shell 16 and the mass 32, so that the mass is frozen in place and will not move.
6. Corrosion developing on the spring 38 changes the response parameters of the switch, which is very undesirable.
7. There is a tendency for corrosion to develop on the the mass 32, on the inner side 39 of the ring 24, and on the tip 29 of the lead 28, so that the switch is permanently open circuited between the leads 12 and 28 or closed contact resistance between the leads 12 and 28 becomes unacceptable.